This invention relates to logging an earth formation to determine the producibility of hydrocarbons contained in that formation. In particular, this invention discloses a means and method for measuring the separate amounts of dispersed and laminated clay in an earth formation.
In well logging one frequently encounters clay minerals contained in sandstone formations. These clay minerals are found in three basic morphologies: dispersed (also called disseminated) clay, laminated clay (also called laminated shale), and structural clay. In the case of dispersed clay, the clay minerals are dispersed throughout the pore space and occupy part of that pore space. Typically, the clay minerals randomly coat the sand grain surfaces with separate, discrete clay particles. This type of clay morphology is usually formed between the sand grains after the sandstone matrix has been deposited. In the case of laminated clay, the clay is usually deposited as a continuous geological layer between geological layers of sand. The clay layers are then subjected to compaction over geological time. These clay laminae are spatially continuous over large distances and can vary in thickness from several microns to many feet thick. Laminated clay also fills the pore space and replaces the matrix proportionally to their volume. The third clay morphology, structural clay, is much less common. Structural clay replaces individual sand grains in the matrix.
The type of clay distribution to a large degree determines the effective porosity, permeability, and productivity of shaly reservoirs. In sufficient quantity, clays can have a serious deleterious effect on the performance of a petroleum reservoir. One important reason for distinguishing between dispersed and laminated clay is the fact that laminated clay is often found between strata of clean (i.e. clay-free) sand. The thickness (thinness) of the laminations may be such that standard logging tools fail to resolve the clay laminations and only a net response from the average clay content from both clean sand and laminated clay is observed. The average clay content can be so large that, if it were dispersed, the petrophysicist would reach a pessimistic conclusion about the producibility of the reservoir; whereas, in fact, the clay-free sand between the clay laminations may have excellent productivity for hydrocarbons. Another advantage of distinguishing between dispersed and laminated clay is that this leads to a recalculation of the hydrocarbon saturation, because hydrocarbons are almost always contained within the sandstone and not within the laminated shale.